Dental calculus, or tartar as it is sometimes called, is a deposit which forms on the surfaces of the teeth at the gingival margin. Supragingival calculus appears principally in the areas near the orifices of the salivary ducts; e.g., on the lingual surfaces of the lower anterior teeth and on the buccal surfaces of the upper first and second molars, and on the distal surfaces of the posterior molars.
Mature calculus consists of an inorganic portion which is largely calcium phosphate arranged in a hydroxyapatite crystal lattice structure similar to bone, enamel and dentin. An organic portion is also present and consists of desquamated epithelial cells, leukocytes, salivary sediment, food debris and various types of microorganisms.
As the mature calculus develops, it becomes visibly white or yellowish in color unless stained or discolored by some extraneous agent. This is undesirable from an aesthetic standpoint.
Mechanical removal of calculus periodically by the dentist is routine dental office procedure. A variety of chemical and biological agents have also been suggested to retard calculus formation or to remove calculus after it is formed. Pyrophosphate salts are chemical agents known to have the ability to retard calculus formation as described, for example, in U.S. Pat. No. 4,999,184, to Parran, Jr. et al., issued Mar. 12, 1991, the disclosures of which are incorporated herein by reference in their entirety.
Dentifrice compositions containing pyrophosphate salts and sodium bicarbonate (aka, baking soda) are also known, having been described, for example, in U.S. Pat. No. 5,180,576, to Winston et al., issued Jan. 19, 1993, the disclosures of which are incorporated herein by reference in their entirety. Obtaining stable dentifrice compositions containing sodium bicarbonate and pyrophosphate effective for inhibiting dental calculus formation, however, presents a processing challenge. Pyrophosphate dissolved in these dentifrice compositions is subsequently likely to recrystallize in the form of tetrasodium pyrophosphate decahydrate crystals. These crystals can grow into large, glass-like particles which have negative aesthetics and also have tartar control efficacy concerns.
U.S. Pat. No. 5,180,576 (incorporated hereinbefore) apparently recognizes this problem and describes avoiding it by a manufacturing process whereby the sodium bicarbonate is used to salt out the alkali metal pyrophosphate. The amount of dissolved pyrophosphate salt is therefore said to be relatively low in the final composition and the remaining pyrophosphate particles are said to be undissolved tetrasodium pyrophosphate decahydrate salted out by the sodium bicarbonate. These undissolved pyrophosphate crystals dissolve very quickly, however, when the composition is diluted with water as occurs during use. This then is just another way of practicing the invention disclosed and claimed, for example, in U.S. Pat. No. 4,684,518, to Parran, Jr. et al., issued Aug. 4, 1987, the disclosures of which are incorporated herein by reference in their entirety.
It has been discovered by the present invention that pyrophosphate/sodium bicarbonate-containing dentifrice compositions which are aesthetically acceptable and efficacious may also be prepared by the simpler process of the present invention. The present invention process, unlike the U.S. Patent 5,180,576 process whereby pyrophosphate is dissolved and then salted out, achieves these benefits through controlling the process conditions to insure that less than about 20% of the pyrophosphate is solubilized at any point in the process mixture. Thus, the present invention process provides advantages versus the prior art, such as the U.S. Pat. No. 5,180,576 process, in terms of simplicity of manufacturing by avoiding the need to control crystallization during salting out of the tetrasodium pyrophosphate decahydrate.
Preferably one or more of three processing conditions are controlled while tetrasodium pyrophosphate salt is added to the mixture. These conditions are the temperature of the mixture, and/or the pH of the mixture, and/or the amount of non-pyrophosphate sodium-containing components added to the mixture.
It is therefore an object of the present invention to provide an easily controlled process for preparing a stable dentifrice composition containing sodium bicarbonate and pyrophosphate salts. An object is also to provide stable dentifrice compositions containing sodium bicarbonate and tetrasodium pyrophosphate salt particles of predetermined and/or consistent size. Another object of this invention is to prepare such stable compositions by a process controlling the processing conditions such that less than about 20% of the total pyrophosphate is dissolved in the composition.
These and other objects of the present invention will become readily apparent from the detailed description which follows.
All percentages and ratios used herein are by weight, and all measurements are made at 25.degree. C., unless otherwise specified.